Photosynthesis
AGRN 176
Were you thinking about where the food came from?
What is the ultimate source of all food on earth?
Autotrophs!
Autotrophs obtain carbon from inorganic sources like carbon dioxide
and energy through either photosynthesis (sunlight)
or chemosynthesis (chemical reactions)
?
Autotrophs are the foundation of all food webs
Food web
Where does the energy come from
at the bottom of the pyramid and what happens to the energy that does not move up to the next level?
AKA Primary Producers
~ 10% of caloric energy is transferred from 1 trophic level to the next higher level
What eventually
happens to the energy that accumulates at each level?
Heterotrophs
(consumers and decomposers)
obtain C and energy from eating living and/or
dead biomass
Which types of organisms are AUTOTROPHS?
Plants
are the
dominant
autotrophs
but some prokaryotes
(bacteria and archaea)
and some protists (algae)
are also autrotrophs
> 80%
of living C!
Distribution of carbon in life on Earth
Are all plants autotrophs?
Carnivorous and parasitic plants are heterotrophs!
Arthropods (mostly insects!) are the dominant consumers of plants!
What is the main purpose of crop production?
➲ Understanding photosynthesis will help YOU accomplish this objective!
Convert solar energy into carbs & other forms of stored energy
Carbohydrates are the dominant form of biological energy storage on planet Earth
Cellulose
is the dominant form of carbohydrate
on planet Earth
Simpler carbohydrates like sugars and starches are also abundant
?
Photosynthesis – the big picture
6 CO2 + 12 H2O + Sunlight
C6H12O6 (glucose) + 6 O2 + 6 H2O
stomata
Where does photosynthesis occur in plants?
➲ All green tissues (primarily leaves)
Primarily mesophyll cells in leaves
– thylakoids
» chlorophyll
http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/14705175/ U3CP5-1_ChloroplastStructu_ksm.jpg
Photosynthesis is a complex multi-step process
Light reaction
➲ Photosystem 1 and Photosystem 2 both capture light energy
➲ Energy from excited electrons is used to drive 3 main reactions:
H2O→O2 (PS2)
ADP + Pi = ATP (PS2)
NADP+ + e- = NADPH (PS1)
PS 1 was discovered before PS 2
PS 2 actually occurs before PS 1
Dark (light independent) reaction
➲ Conversion of carbon dioxide to glucose
Requires ATP and NADPH from light reaction
➲ “Calvin cycle” or “Calvin–Benson–Bassham (CBB) cycle”
The Nobel Prize in Chemistry 1961. NobelPrize.org. Nobel Media AB 2019. Thu. 17 Oct 2019. <https://www.nobelprize.org/prizes/chemistry/1961/summary/>
Calvin cycle = C fixation
enzyme that
captures CO2
Photorespiration
➲ Inefficiency in
C3 photosynthesis
➲ Rubisco sometimes captures O2
➲ Happens when there are low CO2
concentrations
in leaf tissue
☺
☹
C3 vs. C4 photosynthetic pathways
➲
➲
C3 pathway (~90% of all plants)
3 carbon compound molecule produced by first step
RUBISCO captures CO2 (and some O2 = photorespiration ☹)
All cool season crops (wheat, alfalfa, sugarbeet, potatoes) and many warm season crops (soybean, rice, tomatoes)
C4 pathway (~3 % of plants)
4 carbon molecule produced by first step
PEP carboxylase captures CO2
Some warm season plants (corn, sorghum, sugarcane, waterhemp!)
C3
C4
C4 Pathway
Sheaffer and Moncada, 2012
In contrast to the C3 pathway,
the C4 pathway includes an intermediate step PRIOR to the
Calvin cycle
PEP carboxylase
C3 vs C4 – structural differences in leaves
Stoller, 2006
C3
C4
There is a 3rd photosynthetic pathway!
orchids
pineapple
Factors that affect photosynthesis
Lettuce growing in a climate controlled hydroponic system.
Hydroponic crops can grow faster and produce higher yields than crops grown in the field.
Impact of CO2 concentration
➲ Earth’s atmosphere
N2 – 78%
O2 – 21%
CO2 - 0.042%
➲ Movement into leaf (diffusion)
Stomata
Through cell membranes of mesophyll cells
➲ Negative response to limited CO2 (C3)
➲ No response to limited CO2 (C4)
~ 350 ppm when I graduated from college
Riddle for you to solve ☺
Which plants will benefit most
if CO2 concentrations continue to increase?
Acquaah, 2005
Impact of leaf age and health
➲ Photosynthesis declines as leaves age
Where are older leaves in a plant canopy?
➲ Crop diseases negatively affect photosynthesis
Chlorosis = leaves are yellow
Necrosis = leaf tissue is dead
Impact of LAI
➲ LAI = ratio of leaf area to ground area
When LAI < 1,
light is hitting the ground
➲ When is LAI highest during the growing season?
Using a digital LAI meter for measuring LAI
Impact of LIGHT quality, intensity and duration
New WR - Travis Gienger, Anoka MN
Why is it easier to grow giant pumpkins in MN than in IL?
Summer days are longer at higher latitudes!
Should I buy some giant pumpkin seeds?
Impact of temperature/water
➲ Photosynthesis increases with temperature up to a maximum which varies with species
+10C (18 F) – enzyme rate doubles
➲ C3 vs C4 species: How do they differ in response to
high temperature and drought?
Transpiration of water is the cooling system for plants
C3 plants transpire more water per unit of biomass production
C4 plants have a higher optimal temperature range
Impact of nutrients
Liebig’s
law of the minimum
Useful starting point but oversimplification
Nutrients interact!
The basic structure of a chlorophyll molecule is a porphyrin ring with
a central Mg atom.
This is very similar in structure to the heme group found in hemoglobin, except that in heme
the central atom is Fe,
whereas in chlorophyll
it is Mg.
Magnesium is underappreciated as a macronutrient!
Total light energy shining on a leaf
Only ~50% of total energy in sunlight is photosynthetically active radiation (PAR)
Only ~50% of PAR is absorbed by chlorophyll
Only ~30% of energy absorbed by chlorophyll gets converted into glucose
100%
Energy
50%
25%
7.5%
Most of the energy in sunlight is NOT captured by plants
What happens to the glucose produced by the Calvin cycle?
Total glucose produced
~40% of the glucose is lost to respiration
~33% is translocated to roots
~50% ends up in harvested grain
7.5%
Energy
4.5%
3%
1.5%
photosynthetic conversion efficiency
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4453776/
= 5%
These #s are for the whole plant
URBANA, Ill. (WCIA) — The University of Illinois is mourning the passing of an award-winning professor and trailblazer in plant research.
Stephen Long passed away this week at the age of 75 from an extended battle with cancer, the university said in a letter. The Ikenberry Endowed Chair Emeritus of Plant Biology and Crop Sciences at the university, Long was described as a pivotal figure in plant research, leading the field in greater understanding of plant mechanisms, especially photosynthesis.
“His work was characterized by deep insight and expertise, unwavering commitment to tackling some of the most difficult challenges in plant biology, and significant results in agricultural research over a career that spanned five decades,” said Gene Robinson, Director of the Carl R. Woese Institute for Genomic Biology.
https://www.youtube.com/watch?v=W2PQR5RI8CE
This is Dr. Long’s research!
How does energy production/acre/year by solar panels and wind turbines
compare with ethanol from high yield corn?
300 bu/a
x 3 gallons/bu
-------------------
900 gallons
of ethanol
20,000 kWh/a/yr
How many kWh can solar panels produce per acre per year in IL?
In IL, utility-scale solar panels
can produce approximately
300,000 - 450,000 kWh/acre/year.
Illinois Specific Data:
The University of Illinois's Solar Farm,
a 12.3 MW system on 54 acres, produces approximately 20,000 MWh per year, which translates to
370,000 kWh/acre/year.
How many kWh can a wind farm produce per acre per year in IL?
In IL, a utility-scale wind farm
can produce approximately
52,800 - 74,800 kWh/acre/year.
Illinois Specific Data:
Twin Groves Wind Farm (22,000 acres) in McLean County produces 1,162,000,000 kWh per year which translates to 52,818 kWh/acre/year.
This figure is an estimate for the entire wind farm footprint, including the large spaces between turbines (>90% of area) which are often
still used for farming.
The #s on the previous slide for ethanol, solar and wind are all energy OUTPUTS
Energy INPUTS need to be subtracted from OUTPUTS
to calculate NET energy production for all 3 systems
Total Energy OUTPUTS – Total Energy INPUTS
= Life Cycle Analysis which is a topic for a different class
5.5 kWh/m2/day
x 4047 m2/acre
x 100 days
2,225,850 kWh/acre/season
x 1%
22,259 kWh/acre/season
Another way to look at solar energy capture by corn
a little higher than our
high yield ethanol estimate
= total solar energy
Managing for maximum photosynthesis
+ better Rubisco or
other future genetic improvements
in photosynthesis
I like
photosynthesis!